This invention relates to the optimal configuration of the oscillator section and the amplifier sections of free electron or free positron generators of electromagnetic radiation, generally known as free electron lasers (FELs).
Many commercial applications of free electron lasers will require a high peak or average output power. Since the output coupling fraction of oscillators is typically low, the oscillator module of an FEL generates a higher-power intracavity optical beam than can be extracted. Furthermore, the design of oscillators is constrained by the requirement that the intracavity power not exceed the damage threshold of the resonator mirrors. For both of these reasons, a considerable amount of effort has gone into designing and testing FELs which include amplifier sections. In the amplifier, the optical beam which is outcoupled from the oscillator is amplified to high power levels. In contrast to the oscillator case, all of the output from the amplifier is available for the applications, and since the requirement for feedback does not apply to the amplifier, the mirrors may be placed a large distance away, reducing the power density below the damage threshold. This configuration is called the master oscillator-power amplifier (MOPA) configuration.
The first (and only) experimental test of the MOPA configuration was run in a collaborative effort between Stanford and Rocketdyne in the fall of 1988. A design study for the use of the MOPA configuration in the FEL was published in 1988 by A. Bhowmik, et al., "Design concept for a common RF accelerator driven free electron laser master oscillator/power amplifier", Nucl. Instr. & Meth., A272 (1988) 183-186. The experiment used the Mark III accelerator, a Stanford oscillator constructed in 1985 (as described by S.V. Benson, et al. "Status report on the Stanford Mark III IR FEL", Nucl. Instr. & Meth., A272 (1988) 22-28), and a Rocketdyne wiggler constructed in 1987 (as described by A. Bhowmik, et al., "First operation of the Rocketdyne/Stanford FEL", Nucl. Instr. & Meth., A272 (1988) 10-14), which was used as the amplifier. This experiment suffered from a problem caused by the fact that the electron beam passed through the oscillator before it entered the amplifier. This problem comes from the fact that the energy spread of the electron beam is increased by the free electron laser interaction. The gain of the amplifier is reduced by this additional energy spread. This effect was recognized in the design of the MOPA experiments, but action could not be taken to remedy this effect. As a result, data could only be taken with very low oscillator power so that the perturbation of the electron beam energy spread by the oscillator was negligible. The required time gating on the measurement electronics was difficult and complex, and the output power of the amplifier was considerably.
Two alternatives have been proposed in A. Bhowmik, et al., A272 (1988) 183-186, above to solve this problem. The first proposal is to chop up the electron beam with an RF device which would separate alternate micropulses into separate beamlines, one going into the oscillator and the other bypassing the oscillator and leading directly to the amplifier. This is a costly approach which requires a high power RF beam chopper and control electronics, as well as a duplicate beamline to bypass the oscillator.
The second proposal is to add an intracavity optical element to the oscillator which would shut off the oscillator at certain times so that the electron pulses desired to pass through to the amplifier unperturbed would never overlap with laser pulses in the oscillator. This second proposal, although much cheaper than the first, suffers from the grave difficulty that it requires the intracavity use of optically active materials which have significantly lower damage thresholds than the resonator mirrors. This option therefore requires lowering the output power of the oscillator, which in the MOPA configuration would require the use of a higher amplifier gain to obtain the same output power.
What is needed is a method and an apparatus for providing power amplification for a free electron laser which is less costly and more effective than heretofore proposed.